1. Field of the Invention
The present invention relates to an ink jet printer having a function of correcting printing shear based on a printing shear detection pattern which is read by a printing shear detection sensor.
2. Description of the Related Art
Conventionally, an ink jet printer has been used to form an image on a recording paper sheet by discharging ink drops from a recording head of an ink cartridge while reciprocating a cartridge holding member (hereinafter referred to as a carrier) which mounts the ink cartridge thereon in a direction (hereinafter referred to as a main-scanning direction) perpendicular to a direction of conveying the recording paper sheet (hereinafter referred to as a sub-scanning direction). Such ink jet printer having a function of automatically correcting printing shear has been put to a practical use. For example, the function of automatically correcting printing shear is carried out so that a predetermined printing shear detection pattern is formed on a recording paper sheet by the recording head, and the pattern is read by the printing shear detection sensor when attaching the ink cartridge to the carrier, and a timing of controlling the recording head is adjusted on the basis of the read result.
FIG. 6 shows an example of a conventional printing shear detection sensor used for correcting printing shear. The printing shear detection sensor 83 is attached to the carrier. When reciprocating with the carrier in the main-scanning direction, the printing shear detection sensor 83 reads the printing shear detection pattern formed on the recording paper sheet P. The printing shear detection sensor 83 comprises a light emitting device 83a such as LED, a light receiving device 83b such as a photo transistor and two lenses 83c and 83d produced by molding resin or the like.
As shown in the figure, the light emitting device 83a is disposed so that light emitted from the light emitting device 83a may be radiated toward the recording paper sheet P located below the printing shear detection sensor 83 in a direction inclined with respect to the direction vertical to the recording paper sheet P. The light receiving device 83b is oriented to receive the light emitted from the light emitting device 83a which is further reflected on the recording paper sheet P.
The two lenses 83c and 83d are condenser lenses and provided on an optical path from the light emitting device 83a to the recording paper sheet P and an optical path from the recording paper sheet P to the light receiving device 83b, of the light radiated from the light emitting device 83a, respectively. The lens 83c is shaped and disposed so as to refract the light radiated from the light emitting device 83a to converge the light on a predetermined point on the recording paper sheet P as spot light. The lens 83d is shaped and disposed so as to refract the light reflected on the recording paper sheet P and to converge the light from the light receiving device 83b on a detectable area.
The printing shear detection sensor 83 carries out an operation detecting the printing shear detection pattern so that an intensity of the light, which is radiated from the light emitting device 83a and then reflected on the recording paper sheet P, is detected with the light receiving device 83b. That is, the printing shear detection sensor 83 detects the printing shear detection pattern based on the phenomenon that the intensity of the light incident on the light receiving device 83b varies corresponding to presence or absence of the printing shear detection pattern on the light reflection area on the recording paper sheet P.
Hereupon, for example, when directivity of the light radiated from the light emitting device 83a is weak and thus the light expands as it comes closer to the recording paper sheet P or the light reflected on the recording paper sheet P expands as it moves away from the recording paper sheet P, variations in the intensity of the light from the light emitting device 83a incident on the light receiving device 83b may become small, so that it causes the reduction of the detection accuracy of the printing shear detection pattern. In order to prevent this disadvantage, the lenses 83c and 83d are provided at the printing shear detection sensor 83 as described above for converging the light radiated from the light emitting device 83a on the recording paper sheet P and for converging the light reflected on the recording paper sheet P on the light receiving device 83b, thereby enabling the detection of the printing shear detection pattern, surely.
However, as described above, since the lenses 83c and 83d used in the printing shear detection sensor 83 are condenser lenses for surely converging light on one point on the recording paper sheet P or the light receiving device 83b, a molding die for forming the lenses 83c and 83d is expensive because it is particularly and precisely worked and shaped in optical precision, and thereby, the cost of the lenses 83c and 83d is increased. Consequently, the printer also becomes expensive.
On the other hand, Japanese Laid-Open Patent Publication No. 2003-65741 discloses a surface information detecting apparatus for judging whether the recording paper sheet P is embossed paper or high-quality paper by detection surface character of the medium to be detected on the basis of reflected light from the recording paper sheet P. This apparatus has a plurality of lenses for converging light incident on the light receiving device 83b. Thus, this apparatus has a problem similar to that in the above-mentioned case.